JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E07S01, doi:10.1029/2005JE002640, 2006

Overview of the coordinated ground-based observations of during the mission Olivier Witasse,1 Jean-Pierre Lebreton,1 Michael K. Bird,2 Robindro Dutta-Roy,2 William M. Folkner,3 Robert A. Preston,3 Sami W. Asmar,3 Leonid I. Gurvits,4 Sergei V. Pogrebenko,4 Ian M. Avruch,4 Robert M. Campbell,4 Hayley E. Bignall,4 Michael A. Garrett,4 Huib Jan van Langevelde,4 Stephen M. Parsley,4 Cormac Reynolds,4 Arpad Szomoru,4 John E. Reynolds,5 Chris J. Phillips,5 Robert J. Sault,5 Anastasios K. Tzioumis,5 Frank Ghigo,6 Glen Langston,6 Walter Brisken,7 Jonathan D. Romney,7 Ari Mujunen,8 Jouko Ritakari,8 Steven J. Tingay,9 Richard G. Dodson,10 C. G. M. van’t Klooster,11 Thierry Blancquaert,11 Athena Coustenis,12 Eric Gendron,12 Bruno Sicardy,12 Mathieu Hirtzig,12,13 David Luz,12,14 Alberto Negrao,12,14 Theodor Kostiuk,15 Timothy A. Livengood,16,15 Markus Hartung,17 Imke de Pater,18 Mate A´ da´mkovics,18 Ralph D. Lorenz,19 Henry Roe,20 Emily Schaller,20 Michael Brown,20 Antonin H. Bouchez,21 Chad A. Trujillo,22 Bonnie J. Buratti,3 Lise Caillault,23 Thierry Magin,23 Anne Bourdon,23 and Christophe Laux23 Received 17 November 2005; revised 29 March 2006; accepted 24 April 2006; published 27 July 2006.

[1] Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) tracking of the Huygens signal at 2040 MHz, (2) observations of the and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan’s atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial , recovering a vertical profile of wind speed in Titan’s atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission. Citation: Witasse, O., et al. (2006), Overview of the coordinated ground-based observations of Titan during the Huygens mission, J. Geophys. Res., 111, E07S01, doi:10.1029/2005JE002640.

1Research and Scientific Support Department, ESA, ESTEC, Noord- 12LESIA, Observatoire de -Meudon, . wijk, Netherlands. 13Laboratoire de Plane´tologie et de Ge´odynamique, Nantes, France. 2Radioastronomisches Institut, Universita¨t Bonn, Bonn, Germany. 14Observato´rio Astrono´mico de Lisboa, Lisbon, Portugal. 3Jet Propulsion Laboratory, California Institute of , 15NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. Pasadena, California, USA. 16National Center for and Space Education, Washington, 4Joint Institute for VLBI in Europe, Dwingeloo, Netherlands. D. C., USA. 5Australia Telescope National Facility, CSIRO, Epping, Australia. 17European Southern Observatory, Santiago, Chile. 6National Radio Astronomy Observatory, Green Bank, West Virginia, 18Department of Astronomy, University of California, Berkeley, USA. California, USA. 7National Radio Astronomy Observatory, Socorro, New Mexico, USA. 19Lunar and Planetary Laboratory, , Tucson, 8Metsa¨hovi Radio Observatory, Helsinki University of Technology, Arizona, USA. Kylma¨la¨ Finland. 20Division of Geological and Planetary , California Institute of 9Swinburne University of Technology, Hawthorn, Australia. Technology, Pasadena, California, USA. 10Observatorio Astrono´mico Nacional, Alcala´de Henares, Spain. 21Caltech Optical Observatories, California Institute of Technology, 11ESA, ESTEC, TEC Directorate, Noordwjik, Netherlands. Pasadena, California, USA. 22Gemini Observatory, Hilo, Hawaii, USA. Copyright 2006 by the American Geophysical Union. 23Laboratoire EM2C, Ecole Centrale Paris, CNRS-UPR288, Chaˆtenay- 0148-0227/06/2005JE002640 Malabry, France.

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1. Introduction [6] All these observations in various domains of the electromagnetic spectrum were complementary to the Huy- [2] In 2003, the International Astronomical Union Com- gens measurements. The radio astronomy segment proved mission 16 (Physical Study of Planets and ) ‘‘en- especially valuable following the loss of the Cassini channel dorsed astronomical observations of the Saturnian system at A receiver in that it largely recovered the primary goal of the time of the NASA and ESA Cassini/Huygens mission to the Doppler Wind Experiment [Bird et al., 2005; Lebreton the Saturnian system. The attention of the world-wide et al., 2005; Folkner et al., 2006]. Other astronomical astronomical community is drawn to the unique scientific observations provided key information on the atmosphere opportunities presented by the presence of a long-lived and surface properties. Section 2 gives an overview of the orbiting in the Saturnian system and a Titan coordinated set of observations, and section 3 outlines the Probe. Observations of all types, ground- and space-based, scientific results achieved so far. Engineering achievements are encouraged during the course of the mission (nominally are summarized in section 4. 2003–2008), including observations of , the rings, Titan, and the icy satellites.’’ It was therefore decided to support and coordinate, at the level of the Huygens Project 2. Overview of the Ground-Based Observations Scientist Team, a series of ground-based observations at the [7] Many observations of Titan were made during the time of the Huygens mission. The results from the observ- Huygens mission. Here, only the coordinated ground-based ing campaign are presented in this special section. observations are briefly described and put into context. [3] The Huygens mission was carried out successfully on 14 January 2005. An overview of the mission is given by 2.1. Radio Tracking of the Huygens Signal Lebreton et al. [2005], while the first scientific results from all [8] Seventeen radio telescopes listed in Table 1 and experiments are reported by Bird et al. [2005], Fulchignoni et displayed in Figure 1 participated in the monitoring of the al. [2005], Israel et al. [2005], Niemann et al. [2005], carrier signal driven by the DWE ultra-stable oscillator Tomasko et al. [2005], and Zarnecki et al. [2005]. onboard the Huygens Probe and formed the radio astrono- [4] At the time of the mission, no fewer than 17 radio my segment of the mission. This segment consisted of two telescopes were pointed at Titan and tuned to the frequency types of observations: of the Huygens ‘‘channel A’’ carrier signal at 2040 MHz. [9] 1. All 17 radio telescopes participated in Very Long [Lebreton et al., 2005]. An Earth-based radio-tracking effort Base Interferometry (VLBI) observations of the Huygens on this scale was not planned during the original design of Probe. Of these, 15 telescopes were tuned to the Huygens the mission and required major coordination that included channel A carrier frequency of 2040 MHz, while two others dry-run observations in August and November 2004. The did not observe the Probe at this frequency but were Huygens radio astronomy ground-based segment was involved in the overall ‘‘phasing-up’’ the network of radio designed to achieve the following three goals: real-time telescopes by observing the calibrator sources at frequencies detection of the Huygens carrier signal; Doppler tracking as not covering the value of 2040 MHz. The goal of the VLBI an enhancement to the Doppler Wind Experiment (DWE) observations, led by the Joint Institute for VLBI in Europe [Folkner et al., 2006]; acquisition of Very Long Base (JIVE), was to reconstruct the projection of the descent Interferometry (VLBI) data for determining the position of trajectory on the plane of the sky, with an expected linear the Probe in the celestial plane. accuracy of the order of 1 km. [5] Eight large optical observatories participated in coor- [10] 2. Six radio telescopes from the entire network of dinated observations of Titan before, during and after the seventeen participated in Doppler observations of the Huy- Huygens mission. The first objective was to carry out gens Probe, in parallel with the VLBI observations. The scientific observations of Titan in various fields: near- goal of these observations, led by NASA’s Jet Propulsion infrared studies of the atmosphere and of the surface [de Laboratory, was to generate a full two-dimensional charac- Pater et al., 2006; Hartung et al., 2006; M. Hirtzig et al., terization of Titan’s horizontal wind field during the Probe’s Atmospheric and surface features as observed with NAOS/ descent from a combination of the planned Doppler mea- CONICA at the time of the Huygens’ landing, submitted to surements on the Probe-Orbiter and Probe-Earth radio links. Journal of Geophysical Research, 2006 (hereinafter referred The two largest radio telescopes of the network, the NRAO to as Hirtzig et al., submitted manuscript, 2006); A. Negrao R.C. Byrd Green Bank and CSIRO Parkes telescopes were et al., Two-micron spectroscopy of Huygens’ landing site on equipped with NASA Deep Space Network Radio Science Titan with VLT/NACO, submitted to Journal of Receivers. These devices were able to digitally record the Geophysical Research,2006(hereinafterreferredtoas Huygens’ carrier radio signals and detect them in real-time. Negrao et al., submitted manuscript, 2006)], determination Four additional telescopes of the of the zonal wind [Luz et al., 2006; Kostiuk et al., 2006; (VLBA), NRAO Pie Town, Kitt Peak, Owens Valley and stratospheric haze distribution [A´ da´mkovics et al., 2006; de , were equipped with JPL-built PC-based Digital Pater et al., 2006], ethane vertical profile determination Doppler Recorders. (T. A. Livengood et al., High-resolution infrared spectros- [11] Both parts of the radio astronomy segment of the copy of ethane in Titan’s atmosphere in the Huygens epoch, Huygens mission have been built on the heritage of previ- Journal of Geophysical Research, 2006; hereinafter referred ous similar experiments. In particular, the Huygens Doppler to as Livengood et al., submitted manuscript, 2006) and tracking experiment with Earth-based radio telescopes global imaging. The second objective was to detect the evolved from the JPL-led observations of the Probe’s entry into Titan’s atmosphere [Lorenz et al., 2006; mission in the atmosphere of [Folkner et al., de Pater et al., 2006]. 1997a] and earlier observations of Pioneer Venus probes

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Table 1. Radio Telescopes Involved in the Radio Astronomy Segment of the Huygens Missiona Observing Time (ERT/UTC) Institute, Country Diameter, m Start Stop 1. Green Bankb NRAO, USA 100 09:31:10 12:15:00 2. VLBA North Liberty NRAO, USA 25 09:31:10 13:15:00 3. VLBA Fort Davis NRAO, USA 25 09:31:10 13:45:00 4. VLBA Los Alamos NRAO, USA 25 09:31:10 14:00:00 5. VLBA Pie Town NRAO, USA 25 09:31:10 14:15:04 6. VLBA Kitt Peak NRAO, USA 25 09:31:10 14:15:00 7. VLBA Owens Valley NRAO, USA 25 09:30:09 14:49:14 8. VLBA Brewster NRAO, USA 25 09:31:10 14:48:00 9. VLBA Mauna Kea NRAO, USA 25 09:31:10 16:00:00 10. Kashima NIICT, Japan 34 09:31:10 16:00:00 11. Sheshan (Shanghai) NAOC &ShAO, China 25 10:01:10 16:00:00 12. ATCAc Australia 5 Â 22 10:01:10 16:00:00 13. Nanshan(Urumqi)c NAOC, China 25 11:31:10 16:00:00 14. Mopra ATNF, Australia 22 10:10:10 16:00:00 15. Parkesb ATNF, Australia 64 12:26:23 16:00:00 16. Hobart U Tasmania, Australia 26 11:13:10 16:00:00 17. Ceduna U Tasmania Australia 30 10:13:10 16:00:00 aERT/UTC, Earth Received Time in the UTC scale. bProvided real-time detection of the Huygens channel A carrier signal at 2040 MHz. cUsed as phase referencing anchors, but did not observe the carrier signal.

[Counselman et al., 1979]. VLBI tracking of planetary Huygens mission. 32 exposures spanned over 4 nights were spacecraft for navigation was developed in the mid-1970s performed around 14 January 2005. and has been frequently used since then [Thornton and [15] Three telescopes were used to study the atmosphere Border, 2003]. VLBI observations of planetary probes have and surface of Titan in the near-infrared (all using adaptive also been used to measure lunar rotational dynamics [King optics), by virtue of the weak absorption found at et al., 1976; Slade et al., 1977] and winds on Venus certain wavelengths and allowing to probe down to the surface. [Counselman et al., 1979; Preston et al., 1986; Sagdeyev [16] 1. The VLT Yepun telescope observed Titan on 15 et al., 1992]. We note that the VLBI technique offers and 16 January by means of the NAOS/CONICA (NACO) extremely high angular resolution that is inversely propor- instrument. Several modes of NACO were used: (1) narrow- tional to the baseline (distance) between the telescopes band filter imaging, gathering information around 1.3 and involved. At wavelengths of radio domain and baselines 2 micron; (2) spectroscopy from 2.0–2.5 micron; (3) Simul- of up to 10,000 km, the VLBI technique enables a milliarc- taneous Differential Imager (SDI) near 1.6 micron; and second-scale ‘‘sharpness’’ in determination of the position (4) Fabry-Perot imaging in the K-band (2.00–2.18 microns). of a point-like source of emission. Recent improvements in [17] 2. Observations with the Near-Infrared Camera the VLBI technique, including wider recording bandwidths (NIRC2) camera at the KECK II were scheduled on 14, and phase-referencing, were employed for the Huygens 15, 16, and 17 January. The camera was used in high observations, providing much greater sensitivity than pre- angular resolution mode. Several filters were used between vious VLBI observations of space probes. 1.485 and 2.299 mm. [18] 3. The William Herschel Telescope equipped with 2.2. Observations of Titan the NAOMI/OASIS system observed on 10, 19, and [12] The following telescopes (listed in Table 2) observed 22 January, in the range 0.8 to 1 microns. Titan with various scientific objectives: [13] 1. The National Astronomical Observatory of Japan Subaru telescope (Hawaii) was equipped with the NASA Goddard Space Flight Center Heterodyne Instrument for Planetary Wind And Composition, HIPWAC [Kostiuk et al., 2001, 2005]. The goal was to directly measure the magni- tude and direction of Titan’s stratospheric zonal wind by measurement of the Doppler-shift of ethane lines near 12 mm emitted from Titan’s stratosphere. Observations were sched- uled on 13, 14, and 15 January. Poor weather prevented measurements on 13 January, permitted only limited mea- surements on 14 January, but allowed better results on 15 January. Figure 1. Approximate locations of Earth-based radio [14] 2. The UV-Visual Echelle Spectrograph (UVES) on telescopes involved in the radio astronomy segment of the the Very Large Telescope (VLT) was used to measure the Huygens mission. All seventeen telescopes participated in Doppler-shifted spectrum reflected by the Titan’s VLBI observations. Two telescopes shown in red (Green atmosphere in the visible range [Luz et al., 2006]. Zonal Bank and Parkes) participated in real-time detection of the winds in the stratosphere are inferred from such an obser- Huygens Channel A carrier signal at 2040 MHz. The latter vation. Titan was not visible with the VLT at the time of the two plus four VLBA telescopes shown in purple partici- pated in Doppler measurements.

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Table 2. Optical Astronomical Observatoriesa Goal: Attempt to Detect the Time of Probe Goal: Facility/Instrument Wavelength Observation Entry Titan Science Subaru/HIPWAC 12 mm 13, 14, 15 January no zonal wind measurements, ethane profile VLT/UVES 420–620 nm 7, 12, 14, 15 January no zonal wind measurements VLT/NACO 1.2–2.5 mm 18–19 December [2004]; 15, 16 January no atmosphere and range surface characterization VLT/SINFONI 1.45–2.45 mm 28 February no atmosphere and range surface characterization Keck/NIRC2 several 14, 15, 16, 17 January yes atmosphere and filters surface between characterization 1.485 and 2.299 mm WHT/NAOMI-OASIS 0.8–1 mm 10, 19, 22 January no atmosphere and range surface characterization IRTF/TEXES near-IR 14 January yes atmosphere characterization HALE IR and canceled (bad weather) yes visible GEMINI Near-IR canceled (bad weather) yes Observatoire du Pic 1.28 mm 9–17 January no effects on du Midi Titan’s surface of the Saturn’s opposition 14-inch telescope at visible observations no monitoring of New Mexico Skies since 2003, cloud activity Observatory including the night of the Huygens mission HST/STIS visible canceled yes Stellar occultation IR and November 2003 no atmosphere campaign visible characterization aColumn 4 indicates if the planned observation was related to the detection of the Huygens Probe entry (yes or no). Scientific goals related to Titan are given in column 5. Subaru is the National Astronomical Observatory of Japan 8.2-m telescope, located at Mauna Kea (Hawaii). HIPWAC is the NASA Goddard Space Flight Center Heterodyne Instrument for Planetary Wind And Composition. VLT is the European Southern Observatory 8-m Very Large Telescope, located in Chile. UVES is the UV-Visible Echelle Spectrograph mounted on one of the VLT telescope. NACO is an adaptive optics system mounted on the Yepun VLT telescope. SINFONI is the new adaptive optics assisted -field spectrometer mounted on the Yepun VLT unit. Keck is a 10-m telescope located at the summit of Mauna Kea (Hawaii), operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The NIRC2 is an adaptive optics system mounted on the Keck telescope. WHT is the William Herschel Telescope, 4.2 m telescope, located in La Palma (the Canary Islands). OASIS is an adaptive optics system operating first at the CFHT (Hawaii) with PUEO and then at the WHT with NAOMI. IRTF is the NASA Infrared Telescope Facility. IRTF is a 3-m telescope located at the summit of Mauna Kea (Hawaii). TEXES is a high-resolution grating spectrograph mounted on IRTF. HALE is the Palomar 5.1-m telescope, located in north San Diego, California. Gemini is an 8-m optical/infrared telescope located in Hawaii. The Pic du Midi Observatory is located in the French Pyrenees Mountains. 1- and 2-m telescopes are being used for professional reasons. HST is the , and STIS is the Space Telescope Imaging Spectrograph.

[19] Two additional observations deserve to be men- 2003, 2004 and 2005, including the time of the Huygens tioned, even if they were not formally part of the coordi- mission. nated ground-based observations during the Huygens [22] Three articles of this special section deal with obser- mission: vations performed outside the ‘‘Huygens mission window’’: [20] 1. The Pic du Midi (France) telescope was observing A´ da´mkovics et al. [2006] present observations conducted on the surface of Titan. The interest was the study of the effect 28 February 2005 with the new adaptive optics assisted of Saturn’s opposition (13 January 2005) on the surface. integral-field spectrometer SINFONI mounted on the VLT. The observation was carried out in the infrared at Hartung et al. [2006] describe an experiment aiming to map 1.28 micron. solid CO2 ice on the surface, based on data obtained with [21] 2. A simple whole-disk cloud monitoring program NACO/VLT on 18 and 19 December 2004. B. Sicardy et al. was developed using a 14-inch telescope located at New (The two Titan stellar occultations of 14 November 2003, Mexico Skies Observatory. Observations were performed in submitted to Journal of Geophysical Research, 2006; here-

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inversion layer near the 515 ± 5 km altitude level. At that level, the temperature locally increases by 15 K in only 6 km, and the peak value of the gradient dT/dz reaches values as high as +6 K/km. This layer has also been observed by the HASI experiment aboard Huygens, at around 507 ± 15 km. Further work is needed to understand the difference in altitude. 3.2. Zonal Winds [25] The direct Doppler wind observations on Subaru [Kostiuk et al., 2006] and VLT [Luz et al., 2006] are combined with other direct ground-based measurements and measurements from Cassini DWE to retrieve a first altitude profile for zonal winds on Titan from 10 to 450 km altitude (Figure 2). A prograde wind is retrieved throughout and significant increase in wind velocity is seen in the upper stratosphere. Wind speeds deduced from stellar occultations (e.g., Sicardy et al., submitted manuscript, 2006) and from thermal maps by CIRS [Flasar et al., 2005] are in good agreement with the Doppler retrievals. Current dynamical models qualitatively agree with this profile and can be further constrained and improved using the wind altitude profile. Observational and theoretical model results lead to a better understanding of atmospheric Figure 2. Velocity retrievals in Titan’s atmosphere [from dynamics of slowly rotating bodies. Evidence of temporal Kostiuk et al., 2006]: HIPWAC/IRHS [Kostiuk et al., 2006]; and possibly spatial variability as well as the possible probe UVES [Luz et al., 2006]; DWE [Bird et al., 2005]; mm- of Titan’s mesosphere by HIPWAC provide new directions wave [Moreno et al., 2005]; and CIRS [Flasar et al., 2005]. for further studies from Cassini as well as from ground- Different altitudes are probed by the different techniques. based observatories. Results indicate large wind variation with height and [26] An improved wind profile for the middle and strato- possibly with time. The Cassini CIRS retrieved velocities sphere and below with higher temporal resolution (2– plotted have been adjusted to account for 55 m/s wind at 3 seconds) than that presented in the preliminary analysis 10 mbar measured by the Doppler Wind Experiment. The of Bird et al. [2005] is published in this special section pre-Cassini [Yelle et al., 1997] (solid line) and Cassini CIRS [Folkner et al., 2006], with a first high vertical resolution (dashed line) thermal profiles are shown. display and interpretation of the winds near the surface and planetary boundary layer. The main features are (1) the drop inafter referred to as Sicardy et al., submitted manuscript, in wind speed to near zero at an altitude between 65 and 2006) report on the two Titan stellar occultations campaign 75 km; (2) the slow and retrograde wind speed between 1 of November 2003. This observation provided important and 5 km altitude, reversing to slightly prograde at the constraints on the validation of the upper atmosphere model surface; and (3) the 1 m/s eastward surface wind consis- of Titan prior to the Huygens release. tent with the theoretical prediction of a surface gradient- wind less than 2 m/s. 2.3. Attempt to Detect the Probe Entry [27] Zonal wind profiles are deduced at about 250 km [23] Five telescopes (see column 4 of Table 2) were altitude from stellar occultation measurements (Sicardy et devoted to the detection of the emission formed during the al., submitted manuscript, 2006). A strong asymmetry Huygens Probe’s entry into Titan’s atmosphere and due to between the northern and southern hemispheres has dissipation of kinetic energy. Details are given by Lorenz et been found in terms of wind intensity (see Figure 3): about al. [2006]. The planned observations included (1) the NIRC2 215 m/s at 55N, 150 m/s at the equator, and then zero in the camera mounted on the KECK II telescope (Hawaii); (2) the summer (southern) hemisphere. TEXES high-resolution spectrometer on IRTF (Hawaii); (3) the Near InfraRed Imager on the Gemini telescope 3.3. Atmospheric Density and Composition (Hawaii); (4) the Palomar Double Spectrograph on the Hale [28] (Sicardy et al., submitted manuscript, 2006, Figure 8) telescope (California); and (5) the Space Telescope Imaging report on stratospheric density profiles. There is a good Spectrograph (STIS) on board the HST. agreement between the retrieved profiles and Yelle’s engi- neering model predictions in the 400–500 km altitude range [Yelle et al., 1997]. However, a closer inspection shows that 3. What do These Observations Bring to Our the density profiles are about 35% denser (at a given Understanding of Titan? altitude) than Yelle’s profile (or about 15 km higher, for a 3.1. Stratospheric Temperatures given density). Considering the uncertainty domain, this discrepancy remains nevertheless marginal. [24] Sicardy et al. (submitted manuscript, 2006) derived temperature profiles between 400 and 600 km height (see [29] The vertical distribution of ethane is discussed by their Figure 9). The most interesting feature is a sharp Livengood et al. (submitted manuscript, 2006), based on

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Figure 3. (top) Two occultation observed on 14 November 2003 from the same site at Sutherland (South Africa), in two different bands: 0.89 microns at the South Africa Astronomical Observatory (SAAO) and 2.2 microns at the InfraRed Survey Facility of Nagoya University (IRSF). The differences in the central flashes are solely due to the differential extinctions of the hazes, which become more and more transparent at longer IR wavelengths. (bottom) Zonal wind profile derived from the November 2003 central flash analysis (solid black ), compared to the Cassini-CIRS profile (red dash-dots) obtained one year later. The wind profile is also compared with other profiles obtained in the previous years (Sicardy et al., submitted manuscript, 2006). high-resolution infrared spectroscopy performed with the which has a steady gradient through the mesosphere, Subaru telescope and the HIPWAC instrument. The ob- reaches a similar concentration at about 440 km above served morphology of the C2H6 spectroscopic signature the stratopause. differs significantly from earlier observations. The spectros- 3.4. Cloud Activity copy favors a vertical distribution of C2H6 that is enhanced in the mesosphere. An ethane mole fraction of 9.4 ± 2.0 to [30] The Keck II captured Titan some moments after the 10.8 ± 2.4 parts per million by volume (ppmv) is retrieved Huygens Probe reached its target (Figure 4). No clouds for the stratosphere, depending on which of two acceptable were detected on that day. A feeble cloud appeared on mole fraction profiles is assumed. These concentrations 15 January and remained visible through 17 January. The are consistent with earlier retrievals from IR heterodyne bright large southern pole feature observed in Titan’s spectroscopy. An enhanced mole fraction in the meso- atmosphere since 1999 and until recently with adaptive sphere of 120 ± 30 ppmv is found with a profile that optics is not evident either in the VLT/NACO data on assumes differing vertically uniform concentrations in the 15 or 16 January. The brightness observed at the location stratosphere and mesosphere. The other acceptable profile, where it usually appears (at 2.12 micron anyway) is about

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beginning of seasonal change and/or could be related to the large cloud event of October 2004. 3.5. Haze Distribution [32] The tropospheric haze enhancement near the South Pole is confined to latitudes above 40S, and aerosol extinction there is retrieved to be enhanced by a factor of 1.7 relative to the latitude of the Huygens landing site [A´ da´mkovics et al., 2006]. The stratospheric extinction is measured to increase linearly at a rate of 0.65 ± 0.05% per degree latitude from 40S into the Northern mid latitudes. Both of these values have been determined in the context of the Huygens/DISR measurement that demonstrate a con- stant tropospheric haze extinction. The observations pre- sented here reveal that the tropospheric haze enhancement observed in 2001, shortly before the summer solstice at the South Pole (in October 2002), has thinned and is far less prominent than in 2005. However, the interpretation by A´ da´mkovics et al. [2004] of an enhancement specifically near the altitude of the tropopause must be revised in light of the Huygens/DISR measurements, and further observa- tions are necessary to show if indeed there is a preferential altitude in the troposphere at which the extinction enhance- Figure 4. Keck image. Near-infrared surface image of ment occurs. The redistribution of tropospheric haze is a Titan captured with Keck adaptive optics system moments new example of large-scale aerosol dynamics, like the after the Huygens Probe reached its target. The bright and seasonal stratospheric haze asymmetry that has been ob- dark patterns on Titan’s surface may be regions of solid ice served, for example, from the Hubble Space Telescope and of liquid hydrocarbons. [Lorenz et al., 2004] and from the ground [Gibbard et al., 2004]. 10% brighter with respect to the rest of the limb, and since 3.6. Surface this is our intrinsic error bar here for the Fabry-Perot [33] The dark/bright surface contrast analysis at different Interferometer, Hirtzig et al. (submitted manuscript, 2006) wavelengths indicated perhaps the presence of coarser cannot claim a detection. The regular Narrow-Band filters grained frost in the dark areas compared to the bright do not see any hint for such a feeble phenomenon, regions, and/or the presence of additional absorbers in these confirming the lack of cloud activity above the South Pole dark areas, such as NH3 and/or NH4SH frost [de Pater et al., of Titan. On the other hand, the NACO images clearly show 2006; Hirtzig et al., submitted manuscript, 2006]. at several wavelengths (in the 2.12–2.15 micron range) a [34] Surface albedo maps were produced at 2 wave- feature near the south pole but at higher latitudes (60– lengths [A´ da´mkovics et al., 2006, Figure 12], giving com- 70S) on the Western lower side; this feature was reported plementary information on the bright spot detected the by by Hirtzig et al. [2006] as a companion to the main Cassini/VIMS experiment. Bright spots detected by the southern cloud or vortex. At the time of the Huygens Very Large Telescope show a spectral response compatible descent it presents a contrast of 87% with respect to the with methane and water ices (Hirtzig et al., submitted surrounding areas. manuscript, 2006). Surface albedo of the Huygens landing [31] This lack of south polar cloud activity on 14 January site was estimated to be 0.12 at 2.03 microns and 0.02 at 2005 is confirmed by the 14 inch telescope photometry 2.12 microns, very close to the values calculated for dark project and Gemini observations [Schaller et al., 2005]. No areas (Negrao et al., submitted manuscript, 2006). cloud activity at the Huygens landing site near the equator [35]CO2 ice was not detected at the surface [Hartung et has ever been observed in adaptive optics images during the al., 2006; Hirtzig et al., submitted manuscript, 2006]. At past four years of observations. South polar clouds were subearth longitudes 284W and 307W, it was found that a observed in nearly all adaptive optics images from October partial coverage of CO2 ice does not exceed 7% or 14% for 2001 until November of 2004 when south polar cloud bright and dark surface regions, respectively. At 65 mas activity dramatically decreased until at least April 2005 angular resolution, the PSF sampled a surface region of (E. L. Schaller et al., Dissipation of Titan’s south polar 1260 thousand km2. The percentages translate into a maxi- clouds, submitted to Icarus, 2006). This decrease in south mum area of 90 or 180 thousand km2 filled with solid CO2 polar cloud activity occurred following an extremely large ice. The enigmatic bright 5 micron spot at 80W, 25S cloud event in October of 2004, where the south polar described by Barnes et al. [2005] is not covered by the clouds brightened to over fifteen times their typical values longitude range of these observations, but would be easily [Schaller et al., 2006]. The Huygens landing occurred detectable if it were pure solid CO2 ice [Hartung et al., 2006]. during a quiescent period of cloud activity on Titan which The Huygens landing site was specifically observed by the lasted for at least five months. The lack of south polar cloud Very Large Telescope (Figure 5). This gives a broader context activity observed during this time period may be the to the probe measurements (Hirtzig et al., submitted manu-

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[38] 2. Titan astrometry (measurements of the celestial position and parameters of the motion of the planet). [39] Folkner et al. [2006] determined the landing position to be latitude 10.33S, longitude 192.32W (167.68E), based on the landed Doppler data combined with the integrated descent trajectory, in excellent agreement with the initial values published by Lebreton et al. [2005]. 4.2. Real-Time Detection of the Huygens Probe’s Signal [40] On 14 January 2005, the NRAO R.C. Byrd directly detected the Huygens carrier signal at around 10:19:26 Earth Received Time UTC, thus pro- viding invaluable confirmation of the overall state of the mission some 6 hours before data could reach the Earth via the Cassini relay. The detection indicated that (1) the Probe entry had been successful, (2) the pilot chute had been deployed and the back cover had been ejected, (3) the Figure 5. Comparison between the NACO/SDI image and main parachute had been deployed, and (4) the Probe had an image taken by Cassini/ISS, showing the high consis- begun to transmit. This was the first indication that the tency between both measurements. The Cassini image Huygens mission was going to be successful. The CSIRO shows the Huygens landing site map wrapped around Titan, Parkes Telescope also provided a direct detection of the rotated to the same position as the January NACO SDI Huygens carrier signal some two hours later and gave the observations. The Cassini/ISS image is courtesy of NASA, first evidence that the Probe had landed and continued to JPL, Space Science Institute. The colored lines delineate the transmit after landing. Two hours after the first detection, the regions that were imaged by Cassini at differing resolutions. Huygens carrier signal was also detected in the Mark 5 The lower-resolution imaging sequences are outlined in [Whitney, 2003] VLBI data recorded at the GBT. This was blue. Other areas have been specifically targeted for an important diagnostic of the overall performance of the moderate- and high-resolution mosaicking of surface VLBI part of the radio astronomy segment of the mission. features. These include the site where the Huygens Probe 4.3. Express e-VLBI Demonstration in Support has touched down on 14 January 2005 (marked with the to the Huygens Mission yellow x (latitude 10.3S; longitude 167.7E), part of the bright region named (easternmost extent of the [41] Recent developments in information and radio as- area covered), and a boundary between dark and bright tronomy make it possible to transport large regions. Image and text are part of a press release from ESO amounts of data from telescopes to processing centers (24 February 2005). The red color corresponds to an thousands of kilometers apart via optical fiber lines. This atmospheric filter at 1.625 micron, while the blue color kind of data transport enables a modification of the VLBI corresponds to a filter for the surface at 1.600 and technique called e-VLBI [Szomoru et al., 2004]. Its major 1.575 micron. advantage lies in the elimination of recording media and shortening the delay between observations and obtaining results. The ultimate case of e-VLBI is real time VLBI. script, 2006; Negrao et al., submitted manuscript, 2006). The Over the last two decades, near-real-time transfer of narrow- data indicate a high gradient of luminosity in this area. band VLBI tracking data was exploited by NASA DSN for tracking planetary probes and other spacecraft with the 4. Engineering/Science Support Achievements transfer data rates of up to several Mbit/s [Thornton and 4.1. Support in the Determination of the Huygens Border, 2003]. Trajectory and Attitude [42] Since 2003, several observatories involved in Huy- [36] Data from the VLBI and Doppler ground-based radio gens VLBI tracking and JIVE were actively involved in observations of the probe are the key to the determination of wide-band e-VLBI research and development projects, the probe trajectory and other dynamical characteristics. including various demonstration tests with the transfer data Measurements of both the Probe plane of sky motions, from rate of several hundreds Mbit/s. It was decided to apply the the VLBI data, and the Probe-Earth relative velocities, from e-VLBI technique for a subset of Huygens VLBI tracking the Doppler observations, are being used. The VLBI data data, using high speed transmission and near real-time analysis indicates that the probe trajectory in the plane of processing. Of the 17 radio telescopes listed in Table 1, the sky will be determined to an accuracy of about 1 km two (the CSIRO Parkes Telescope and Mopra, both [Pogrebenko et al., 2004]. The radio data set will also operated by the Australia Telescope National Facility, provide in the near future additional pieces of information: ATNF), were chosen for the Huygens e-VLBI demonstra- [37] 1. Measurements of the carrier signal frequency are tion. Huygens tracking at Parkes and Mopra was conducted being used to constrain some attitude parameters of the using the LBA’s hard-disk recording system, based on the Huygens Probe during its descent, namely the Probe pen- Metsa¨hovi VERSUSI input cards [Dodson et al., 2004]. dulum motion and spin. These parameters are needed for a Immediately after completion of the Huygens observations careful analysis of the scientific payload data set. (late at night, local time in Australia), a charter plane flew

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Table 3. Importance of Ground-Based Observations in Coordination With in Situ Planetary Missions Topic Examples Selected References Preparation of planetary Target selection of cometary missions missions Scientific support during Study of Titan’s atmosphere Griffith et al. [2005] the mission Coustenis et al. [2003, 2005] Sicardy et al. (submitted manuscript, 2006) Relatively low cost of ground observations to space missions Large science returns of a Huygens this special section joint effort Meech et al. [2005] Extended temporal 1. seasonal change in Lorenz et al. [2004] coverage Titan’s haze 1992–2002 Coustenis et al. [2001] from Hubble Space Hirtzig et al. [2005] Telescope observations 2. study of stratospheric Luz et al. [2006] zonal winds Kostiuk et al. [2006] Extended spatial or zonal wind measurements Luz et al. [2006] global coverage with altitude coverage Kostiuk et al. [2006] complementary to the Huygens DWE results Different wavelength range near-infrared observations Lellouch et al. [2004] above the Huygens DISR Coustenis et al. [2005] IR spectrometer limit Hirtzig et al. [2006, submitted manuscript, 2006] Negrao et al. (submitted manuscript, 2006) de Pater et al. [2006] A´ da´mkovics et al. [2006] Technique only possible 1. VLBI radio tracking of Lebreton et al. [2005] from Earth. Huygens probe and Preston et al. [1986] Venus Balloons to Sagdeev et al. [1990] determine plane-of-sky probe motions (e.g., for wind determination) 2. Doppler radio tracking of Bird et al. [2005] Huygens and Galileo probes Folkner et al. [1997a] to provide Earth-relative Folkner et al. [2006] probe velocity (e.g., for wind determination) 3. Doppler and range radio Folkner et al. [1997b] tracking of Mars Pathfinder and Viking landers to measure Mars rotational irregularities (e.g., for Mars interior structure and seasonal CO2 deposition) 4. radar evidence for liquid Campbell et al. [2003] surfaces on Titan Support in case of failure Huygens radio astronomy Bird et al. [2005] segment following loss of Lebreton et al. [2005] Channel A Folkner et al. [2006] Necessary to achieve the Deep Impact mission Meech et al. [2005] science objectives of a given planetary mission Information on Lorenz et al. [2006] aerothermodynamics during Magin et al [2006] any Caillault et al. [2006] Public outreach, amateur Deep Impact and community Cassini-Huygens missions over the triangle Mopra – Parkes – Sydney to collect and data processor in Dwingeloo, Netherlands. The total amount deliver disks with the recorded data to the ATNF Head- of about 900 Gbits was transmitted with the average data quarters in Epping. From there, two 15-min-long data seg- rate of about 300 Mbit/s. In the next step, the data were ments recorded on one of the background celestial calibrator reformatted remotely at the Helsinki University of Technol- sources (a quasar) with a data rate of 512 Mbit/s, were ogy from the ‘‘native’’ format for the ATNF telescope to the transmitted via fiber optic cables over the Pacific, across Mark 5 standard, suitable for correlation at JIVE. Another North America and further on over the Atlantic to the JIVE 0.5 hours were needed for JIVE staff to detect the interfer-

9of12 E07S01 WITASSE ET AL.: TITAN OBSERVATIONS DURING HUYGENS MISSION E07S01 ometric ‘‘fringes’’ (response) on the baseline Parkes – provided (or will provide) useful information on the trajec- Mopra. This detection was achieved about 13 hours after tory of the Huygens probe, which is very important for the completion of the observations of the Huygens Probe at data interpretation of the probe scientific data set. The real Parkes and Mopra. time detection of the probe carrier signal during descent [43] Interferometric fringes on the calibrator source indicated that the probe survived the entry and started obtained in the Huygens e-VLBI demonstration early in transmitting data. No positive detection of the probe entry the morning of 15 January coupled with the detection of the was reported. However, lessons learned are published in this Huygens channel A carrier signal at GBT and Parkes using special section. Comprehensive studies on heat fluxes and both RSR and VLBI data acquisition during and soon after heat loads were carried out and will be very valuable for the mission on 14 January, provided a solid proof that the future planetary probe entries. Finally, these observations goal of the radio astronomy segment of the Huygens represented a ‘‘down-to-Earth’’ aspect of the mission of mission would be achieved. prime interest to the media and general public. [44] The ‘‘next morning’’ result of the express processing [48] The recent Deep Impact event is another example of of data from the radio astronomy segment of the Huygens coordinated ground-based observations [Meech et al., mission was an efficient verification of the correctness of 2005]. In this case seven Earth-orbiting spacecraft, a the overall setup of the experiment, allowing the team to large number of telescopes and the ESA comet chaser conclude, that the science goal of the experiment would be were observing comet 9P/Tempel 1 when hit by achieved in the course of full data processing. It also the impactor. demonstrated a high potential of similar high data rate [49] The utility of ground-based (or Earth-based) obser- VLBI to support navigation of future planetary missions. vations is summarized in Table 3. Future opportunities for coordinated, supporting and complementary observations to 4.4. Attempt to Detect the Probe’s Entry space missions include: Cassini in the Saturnian system [45] Because of the STIS failure in August 2004, the HST (end of nominal mission: mid 2008), at Mars observation was canceled. In addition, because of the (mission extended until October 2007), at cloudy weather in California and strong winds at the Mauna Venus (orbit insertion on 11 April 2006, end of nominal Kea summit, only the two observations with the Keck II and mission in October 2007), Chandrayaan-1, Selene and IRTF facilities were successful. However, no positive de- Chang’e-1 around the Moon (2007–2009), Messenger at tection of the Probe entry was reported. No signal above Venus (flyby) and Mercury, Bepi-Colombo at Mercury, 0.8 mJ at the wavelength of 1.68 mm has been detected [de Rosetta (Mars flyby in February 2007, asteroid flybys in Pater et al., 2006]. This does not permit to put significant 2008 and 2010, at comet 67P/Churyumov-Gerasimenko constraints on the emission. Lorenz et al. [2006] provides all 2014–2015), at Jupiter in 2007 and at Pluto the details, and draws lessons learned for future observa- in 2015. tions of Probe’s entries. [50] The results presented in this special section represent [46] The modeling of the Huygens entry flux was the goal the achievements one year after Huygens. It is clear that a of an extensive set of studies, which took place in 2004 significant part of the data is still being analyzed, and we [Walpot et al., 2005]. Two articles in this special section expect many more scientific results in the coming months deal with such studies. Magin et al. [2006] describe a model and years. that predicts the population of excited electronic states of the CN and N2 molecules, in order to assess non-equilibrium [51] Acknowledgments. The decision to support and coordinate a radiation effects. Caillault et al. [2006] present radiative series of optical ground-based observations was taken at the time of the heat flux predictions for the Probe entry. The most intense Huygens Science Working Team in October 2003 in Graz. We thank all participants, especially Athena Coustenis and Dennis Matson, for their emission originates from CN in the violet range. This study support. We thank the observatory directors and telescope allocation panels evaluates the role of the self-absorption by the plasma in for their endorsement of all these observations. We thank the European reducing the total emission, and the effect of the chemical funded network EUROPLANET for its support. This research was carried out in part at the Jet Propulsion Laboratory, California Institute of composition on the radiative heating. Technology, under contract with NASA. We appreciate the support pro- vided by the National Radio Astronomy Observatory (NRAO) and the Australia Telescope National Facility (ATNF). NRAO is operated by 5. Conclusion Associated Universities, Inc., under a cooperative agreement with the NSF. The ATNF, managed by the Commonwealth Scientific and Industrial [47] The Earth-based observations performed during or Research Organization (CSRIO), The Joint Institute for VLBI in Europe, is around the Huygens mission proved to be of fundamental funded by the national research councils, national facilities and institutes of importance: VLBI and Doppler measurements from the Netherlands (NWO), the United Kingdom (PPARC), Italy (INAF), Sweden radio astronomy segment were essential in providing mea- (Onsala Space Observatory, National Facility), Spain (IGN), Germany (MPIfR), and China (National Astronomical Observatories, CAS). The surements of the Titan wind field during the Probe’s Huygens e-VLBI demonstration would have been impossible without the descent, which would have otherwise been lost because of efforts of the networking community, who on very short notice provisioned the telecommunications problem. Observations with adap- a dedicated light path between Australia and JIVE. We thank George McLaughlin, Steve Maddocks, Mark Prior, and Alan Cowie (AARNet),- tive optics provided coverage of Titan’s hemisphere where Shaun Amy (CSIRO), Craig Russell (CeNTIE), Herve´ Guy and Damir Huygens landed. The zonal wind was measured at different Pobric(Canarie), Bill Mar (Pacific Northwest GigaPoP), Geoff Lakeman altitudes by various methods, ideally complementing the (University of Washington), Caroline Carver (MANLAN), and Dennis Doppler Wind Experiment. Other scientific results were Paus(SURFnet). Part of this work was funded through grants AST-0205893 from the National Science Foundation and NNG05GH63G from NASA to achieved concerning stratospheric temperatures, density the Univ. of California, Berkeley. David Luz acknowledges financial and composition, cloud activity, haze distribution, surface support from Fundac¸a˜o para a Cieˆncia e a Tecnologia, Portugal (fellowship albedo and composition. VLBI and Doppler measurements PRAXIS XXI/BPD/3630/2000 and project POCI/CTE-AST/57655/2004). Alberto Negra˜o is supported by the FCT Ph.D. scholarship SFRH/BD/

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8006/2002. HIPWAC measurements were supported by the NASA Plan- Kostiuk, T., T. A. Livengood, T. Hewagama, G. Sonnabend, K. E. Fast, etary Astronomy Program. Olivier Witasse thanks Sushil Atreya for useful K. Murakawa, A. T. Tokunaga, J. Annen, D. Buhl, and F. Schmu¨lling suggestions. (2005), Titan’s stratospheric zonal wind, temperature, and ethane abun- dance a year prior to Huygens insertion, Geophys. Res. Lett., 32, References L22205, doi:10.1029/2005GL023897. ´ Kostiuk, T., et al. (2006), Stratospheric global winds on Titan at the time of Ada´mkovics, M., I. de Pater, H. G. Roe, S. G. Gibbard, and C. A. Huygens descent, J. Geophys. Res., doi:10.1029/2005JE002630, in press. Griffith (2004), Spatially-resolved spectroscopy at 1.6 mm of Titan’s Lebreton, J.-P., et al. (2005), An overview of the descent and landing of the atmosphere and surface, Geophys. Res. Lett., 31, L17S05, doi:10.1029/ Huygens probe on Titan, Nature, 438, 758–764. 2004GL019929. ´ Lellouch, E., B. Schmitt, A. Coustenis, and J.-G. Cuby (2004), Titan’s Ada´mkovics, M., I. de Pater, M. Hartung, F. Eisenhauer, R. Genzel, and 5-micron lightcurve, Icarus, 168, 209–214. A. Griffith (2006), Titan’s bright spots: Multiband spectroscopic mea- Lorenz, R. D., P. H. Smith, and M. T. Lemmon (2004), Seasonal change in surement of surface diversity and hazes, J. Geophys. Res., 111, Titan’s haze 1992 – 2002 from Hubble Space Telescope observations, E07S06, doi:10.1029/2005JE002610. Geophys. Res. Lett., 31, L10702, doi:10.1029/2004GL019864. Barnes, J. W., et al. (2005), A 5-Micron-bright spot on Titan: Evidence for Lorenz, R. D., et al. (2006), Huygens entry emission: Observation cam- surface diversity, Science, 310, 92–95. paign, results, and lessons learned, J. Geophys. Res., 111, E07S11, Bird, M. K., et al. (2005), The vertical profile of winds on Titan, Nature, doi:10.1029/2005JE002603. 438, 800–802. Luz, D., et al. (2006), Characterization of zonal winds in the stratosphere of Caillault, L., et al. (2006), Radiative heating predictions for Huygens entry, Titan with UVES: Ii: Observations coordinated with the Huygens entry, J. Geophys. Res., doi:10.1029/2005JE002627, in press. J. Geophys. Res., doi:10.1029/2005JE002617, in press. Campbell, D., et al. (2003), Radar evidence for liquid surfaces on Titan, Magin, T. E., L. Caillault, A. Bourdon, and C. O. Laux (2006), Nonequili- Science, 302(5644), 431–434. brium radiative heat flux modeling for the Huygens entry probe, J. Geo- Counselman, C., S. Gourevitch, R. King, G. Pettengill, I. Shapiro, R. Miller, phys. Res., 111, E07S12, doi:10.1029/2005JE002616. J. Smith, R. Prinn, R. Ramos, and P. Leibrecht (1979), Wind velocities on Meech, K. J., et al. (2005), Deep impact: Observations from a worldwide Venus: Vector determination by radio interferometry, Science, 230, 805– Earth-based campaign, Science, 310, 265–269. 806. Moreno, R., A. Marten, and T. Hidayat (2005), Interferometric measure- Coustenis, A., E. Gendron, O. Lai, J.-P. Veran, J. Woillez, M. Combes, ments of zonal winds on Titan, Astron. Astrophys., 437, 319–328. L. Vapillon, Th. Fusco, L. Mugnier, and P. Rannou (2001), Images of Niemann, H. B., et al. (2005), The abundances of constituents of Titan’s Titan at 1.3 and 1.6 microns with adaptive optics at the CFHT, Icarus, atmosphere from the GCMS instrument on the Huygens probe, Nature, 154, 501–515. 438, 779–784. Coustenis, A., A. Salama, B. Schulz, S. Ott, E. Lellouch, T. H. Encrenaz, Pogrebenko, S. V., L. I. Gurvits, R. M. Campbell, I. M. Avruch, J.-P. D. Gautier, and H. Feuchtgruber (2003), Titan’s atmosphere from ISO Lebreton, and C. G. M. van’t Klooster (2004), VLBI tracking of the mid-infrared spectroscopy, Icarus, 161(2), 383–403. Huygens probe in the , in Proceedings of the Inter- Coustenis, A., M. Hirtzig, E. Gendron, P. Drossart, O. Lai, M. Combes, and national Workshop "Planetary Probe Atmospheric Entry and Descent A. Negrao (2005), Maps of Titan’s surface from 1 to 2.5 micron, Icarus, Trajectory Analysis and Science", Lisbon, Portugal, 5–9 October 177, 89–105. ´ 2003, Eur. Space Agency Spec. Publ., ESA-SP 544, 197–204. de Pater, I., M. Ada´mkovics, A. H. Bouchez, M. E. Brown, S. G. Gibbard, Preston, R. A., et al. (1986), Determination of Venus winds by ground- F. Marchis, H. G. Roe, E. L. Schaller, and E. Young (2006), Titan ima- based radio tracking of the VEGA balloons, Science, 231, 1414–1416. gery with Keck adaptive optics during and after probe entry, J. Geophys. Sagdeev, R. Z., et al. (1990), Measurements of the dynamics of air mass Res., 111, E07S05, doi:10.1029/2005JE002620. motion in the Venus atmosphere with balloon probes—VEGA Project, Dodson, R., S. Tingay, C. West, C. Phillips, A. Tzioumis, J. Ritakari, Sov. Astron. Lett., 16, 357. and F. Briggs (2004), The Australian experience with the PC-EVN Sagdeyev, R. Z., et al. (1992), Differential VLBI measurements of the recorder, in Proceedings of the 7th European VLBI Symposium, edited Venus atmosphere dynamics by balloons—VEGA Project, Astron. Astro- by R. Bachiller et al., p. 253, Observ. Astron. Nacl., Toledo, Spain. phys., 254, 387. Flasar, F. M., et al. (2005), Titan’s atmospheric temperatures, winds, and Schaller, E. L., M. E. Brown, H. G. Roe, A. H. Bouchez, and C. A. Trujillo composition, Science, 308, 975–978. (2005), Cloud activity on Titan during the Cassini mission, Proc. Lunar Folkner, W. M., et al. (1997a), Earth-based radio tracking of the Galileo Planet. Sci. Conf. 36th, abstract 1989. probe for Jupiter wind estimation, Science, 275, 644–646. Schaller, E. L., M. E. Brown, H. G. Roe, and A. H. Bouchez (2006), A large Folkner, W. M., et al. (1997b), Interior structure and seasonal mass redis- cloud outburst at Titan’s south pole, Icarus, 182, 224–229. tribution of Mars from radio tracking of Mars Pathfinder, Science, 278, Slade, M. A., R. A. Preston, A. W. Harris, L. J. Skjerve, and D. J. Spitzmesser 1749–1752. (1977), ALSEP-Quasar Differential VLBI, Moon, 17, 133. Folkner, W. M., et al. (2006), Winds on Titan from ground-based tracking Szomoru, A., A. Biggs, M. Garrett, H. J. van Langevelde, F. Olnon, of the Huygens probe, J. Geophys. Res., 111, E07S02, doi:10.1029/ Z. Paragi, S. Parsley, S. Pogrebenko, and C. Reynolds (2004), From 2005JE002649. truck to optical fibre: The coming-of-age of eVLBI, in Proceedings Fulchignoni, M., et al. (2005), In situ measurements of the physical char- of the 7th European VLBI Symposium, edited by R. Bachiller et al., acteristics of Titan’s atmosphere, Nature, 438, 785–791. p. 257, Observ. Astron. Nacl., Toledo, Spain. Gibbard, S. G., I. de Pater, B. A. Macintosh, H. G. Roe, C. E. Max, E. F. Thornton, C. L., and J. S. Border (2003), Radiometric Tracking Techniques Young, and C. P. McKay (2004), Titan’s 2 mm surface albedo and haze for Deep-Space Navigation, John Wiley, Hoboken, N. J. optical depth in 1996–2004, Geophys. Res. Lett., 31, L17S02, Tomasko, M. G., et al. (2005), Rain, winds and haze during the Huygens doi:10.1029/2004GL019803. probe’s descent to Titan’s surface, Nature, 438, 765–778. Griffith, C., et al. (2005), Observations of Titan’s mesosphere, Astrophys. Walpot, L., L. Caillault, C. O. Laux, R. Molina, and T. Blancquaert (2005), J., 629(1), L57–L60. Huygens entry heat flux prediction, paper presented at 3rd International Hartung, M., T. M. Herbst, C. Dumas, T. Owen, and A. Coustenis (2006), Planetary Probe Workshop (IPPW-3), Am. Inst. of Aeronaut. and Astro- Limits to the abundance of surface CO2 ice on Titan, J. Geophys. Res., naut., Attica, , July. 111, E07S09, doi:10.1029/2005JE002642. Whitney, A. R. (2003), Mark 5 disk-based Gbps VLBI data system, in New Hirtzig, M., A. Coustenis, O. Lai, E. Emsellem, A. Pecontal-Rousset, Technology in VLBI, edited by Y. C. Minh, ASP Conf. Ser. 306, p. 123, P. Rannou, A. Negrao, and B. Schmitt (2005), Near-infrared study Astron. Soc. of the Pac., San Francisco, Calif. of Titan’s resolved disk in spectro-imaging with CFHT/OASIS, Pla- Yelle, R. V., D. Strobel, E. Lellouch, and D. Gautier (1997), Engineering net. Space Sci., 53, 535–556. models for Titan’s atmosphere, in Huygens: Science, Payload and Mis- Hirtzig, M., A. Coustenis, E. Gendron, P. Drossart, A. Negrao, M. Combes, sion, edited by J.-P. Lebreton, Eur. Space Agency Spec. Publ., ESA-SP O. Lai, P. Rannou, S. Lebonnois, and D. Luz (2006), Monitoring atmo- 1177, 243–256. spheric phenomena on Titan, Astron. Astrophys., in press. Zarnecki, J. C., et al. (2005), A soft solid surface on Titan as revealed by the Israel, G., et al. (2005), Complex organic matter in Titan’s atmo- Huygens Surface Science Package, Nature, 438, 792. spheric aerosols from in situ and analysis, Nature, 438, 796–799. ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ King, R. W., C. C. Counselman III, and I. I. Shapiro (1976), Lunar M. A´ da´mkovics and I. de Pater, Department of Astronomy, University of dynamics and selenodesy: Results from analysis of VLBI and laser data, California, 601 Campbell Hall, Berkeley, CA 94720, USA. J. Geophys. Res., 81, 6251–6256. S. W. Asmar, B. J. Buratti, W. M. Folkner, and R. A. Preston, Jet Kostiuk, T., K. Fast, T. A. Livengood, T. Hewagama, J. Goldstein, Propulsion Laboratory, California Institute of Technology, Pasadena, CA F. Espenak, and D. Buhl (2001), Direct measurement of winds on 91109. USA. Titan, Geophys. Res. Lett., 28, 2361–2364.

11 of 12 E07S01 WITASSE ET AL.: TITAN OBSERVATIONS DURING HUYGENS MISSION E07S01

I. M. Avruch, H. E. Bignall, R. M. Campbell, M. A. Garrett, L. I. Gurvits, F. Ghigo and G. Langston, National Radio Astronomy Observatory, P.O. S. M. Parsley, S. V. Pogrebenko, C. Reynolds, A. Szomoru, and H. J. van Box 2, Rt. 28/92, Green Bank, WV 24944, USA. Langevelde, Joint Institute for VLBI in Europe, P.O. Box 2, 7990 AA, M. Hartung, European Southern Observatory, Alonso de Cordova 3107, Dwingeloo, Netherlands. Santiago 19, Chile. M. K. Bird and R. Dutta-Roy, Radioastronomisches Institut, Universita¨t T. Kostiuk, NASA Goddard Space Flight Center, Greenbelt, MD 20771, Bonn, Auf dem Hu¨gel 71, D-53125 Bonn, Germany. USA. T. Blancquaert and C. G. M. van’t Klooster, ESA, ESTEC, TEC J.-P. Lebreton and O. Witasse, Research and Scientific Support Directorate, Noordwjik, 2200 AG, Netherlands. Department, ESA, ESTEC, Noordwijk, 2200 AG, Netherlands. (owitasse@ A. H. Bouchez, Caltech Optical Observatories, California Institute of rssd.esa.int) Technology, MS 150-21, Pasadena, CA 91125, USA. T. A. Livengood, National Center for Earth and Space Science Education, A. Bourdon, L. Caillault, C. Laux, and T. Magin, Laboratoire EM2C, Washington, DC 20036, USA. Ecole Centrale Paris, CNRS-UPR288, Grande Voie des Vignes, F-92295 R. D. Lorenz, Lunar and Planetary Laboratory, University of Arizona, Chaˆtenay-Malabry, France. Tucson, AZ 85721, USA. W. Brisken and J. D. Romney, National Radio Astronomy Observatory, A. Mujunen and J. Ritakari, Metsa¨hovi Radio Observatory, Helsinki P.O. Box 0,1003 Lopezville Road, Socorro, NM 87801, USA. University of Technology, Metsa¨hovintie 114, FIN-02540 Kylma¨la¨, M. Brown, H. Roe, and E. Schaller, Division of Geological and Planetary Finland. Sciences, California Institute of Technology, MS 150-21, Pasadena, CA C. J. Phillips, J. E. Reynolds, R. J. Sault, and A. K. Tzioumis, Australia 91125, USA. Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 1710, A. Coustenis, E. Gendron, M. Hirtzig, D. Luz, A. Negrao, and B. Sicardy, Australia. LESIA, Observatoire Paris-Meudon, F-92195 Meudon Cedex, France. S. J. Tingay, Swinburne University of Technology, P.O. Box 218, R. G. Dodson, Observatorio Astrono´mico Nacional, Apartado 112, 28803 Hawthorn 3122, Australia. Alcala´ de Henares, Spain. C. A. Trujillo, Gemini Observatory, Hilo, HI 96720, USA.

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